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The world's oceans may not be as large as scientists thought for the purposes of reining in global warming.A paper published in the Jan. 10 issue of the journal Science suggests that climate models could be overestimating by as much as 60 percent the amount of carbon dioxide the oceans can absorb.Because the ocean is a large "carbon sink," it processes much of the carbon released into Earth's atmosphere. But if the absorption, or "uptake," of carbon dioxide is less than estimated, global climate change could be approaching a lot faster and be hotter than anticipated.Modeling the climate is a complex and difficult task. Few of the variables stand still long enough for scientists to get a clear shot.Critics point to those shifting variables to deny global warming or to argue against counteracting its effects, but mathematical models right now are the only means of prediction.Ben McNeil, a postdoctoral research associate at Princeton University who conducted the ocean research, said, "The only way we have of looking into the future is via models, and those models need validating. Now we have a measurement of ocean uptake of carbon dioxide, and we can compare it with the models."Based on the data he and his associates have collected, the models "are currently overestimating the ocean's ability to take up excess carbon from the atmosphere. If they are overestimating that, they are probably underestimating the levels in the atmosphere," he said.Unless the carbon is absorbed by some other portion of the biosphere, the planet's climate may heat up faster than models predict.Rik Manninkhof, an oceanographer with the National Oceanic and Atmospheric Administration's oceanographic and meteorological laboratory in Miami, said oceans are believed to take up one-third of the human-caused carbon dioxide. "If that figure is low, the atmospheric CO2 could be higher in the future," he said. "We believe that CO2 increases temperature, so we'll get faster temperature rise."Mr. Manninkhof is quick to point out, though, that the result is not inevitable. Other factors could intervene. A warmer atmosphere could increase biological activity in the oceans support more plankton, for instance which could increase carbon uptake. Another part of the biosphere also could have increased uptake."If you consider a warming scenario, the ocean's ability to take up carbon will change. You would think it would become less able," said Keith Lindsay, a scientist and climate modeler for the National Center for Atmospheric Research in Boulder, Colo.Mr. Lindsay noted that warm water holds less carbon than cold water. "And another one of the predictions is that the ocean becomes more stratified, temperaturewise," he said. "There is less mixing. The portion that can absorb carbon is shallower because the deeper ocean becomes inaccessible to the atmosphere."However, he added, increased biological activity could absorb more carbon, then take it to the depths. "That's one pathway to export carbon to the deep ocean sometimes referred to as the 'biological pump,'" he said.The McNeil study in Science used an analogy to examine ocean carbon uptake. The researchers looked at how well the ocean absorbs chlorofluorocarbons (CFCs), gases that for many years acted as propellants for everything from deodorants to furniture waxes. CFCs were banned because they were destroying the planet's fragile ozone layer, the first line of defense against harmful solar radiation.One problem with measuring carbon uptake directly is that carbon dioxide exists in the ocean naturally but receives a contribution from industrial and human sources. CFCs do not exist naturally in the ocean. They were first produced in the 1940s and have entered the oceans only since then.CFCs and carbon dioxide are different, but comparison of computer models with true observations indicates the analogy works well. "We have quite a bit of confidence in the technique," Mr. McNeil said.In the past, ocean uptake of carbon dioxide was estimated using observations of how well the isotope carbon-14 was absorbed. Carbon-14, which is radioactive, was produced by atmospheric atomic bomb tests in the 1940s and 1950s."They followed the penetration of carbon-14 into the oceans and got the original estimates," said Mr. Manninkhof at NOAA. "By some creative methods [they also have] measured the anthropogenic CO2 since the industrial revolution."Mr. Manninkhof said the McNeil study, although intelligent and important, is not definitive. "I would be a bit cautious," he said. "There are some caveats for their method."The main caveat is the analogy. Carbon dioxide started to increase in the late 1700s with the onset of industry. It increased dramatically with the success of the era. CFCs were developed in World War II, "so by the time the CFCs had started increasing, half the CO2 was already there. So they have to make some creative assumptions about that particular time period. They also have to assume that the ocean is in a steady state. And they acknowledge that they don't get the biological component."The oceans have a large influence on climate, and climate has a large influence on the oceans, but the dynamic is poorly understood.For example, different portions of the oceans behave differently. "One of the least understood," said Mr. Lindsay of NCAR, "is the southern ocean." That's a lot of water.Climate models contain many uncertainties but do give warnings. Critics say it makes no sense to take large economic risks based on these uncertainties but rarely point out that their economic models are no more reliable.Mr. Manninkhof said, "We're a bit skeptical about how much science drives policy, but we hope our research is taken into account."